1. Field of the Invention
The invention relates generally to the field of polarized color light sources, such as light engines for viewing computer and video displays directly or by projection. More particularly, the invention relates to a light source that recovers rejected polarization states and colors to enhance brightness.
2. Description of the Related Art
Many displays used in projection and direct viewing systems operate on the basis of polarization. Such displays include reflective displays such as LCoS (Liquid Crystal on Silicon), super twisted nematic (STN), and ferroelectric (FLC) as well as transmissive displays, such as thin film transistor (TFT), poly-silicon (p-si), and Silicon-on-Insulator (SOI). These displays can produce a high resolution image by changing the polarization state upon reflection or transmission of incident light.
In an LCoS display, for example, in the dark state, a pixel reflects all light with substantially no change in polarization. In the bright state, the pixel rotates the polarization state of reflected incident light to the corresponding orthogonal state. By illuminating the display with polarized light and then filtering out nearly all reflected or transmitted light of that polarization, the display image can be viewed by the human eye or projected onto a viewing screen.
Because most conventional low cost light sources produce light with mixed polarization states, the light is typically analyzed by a PBS (Polarizing Beam Splitter). Light of one polarization direction propagates through the PBS, while light of the orthogonal polarization direction is reflected by the PBS. Another common approach is to use a polarizing filter that absorbs light of one polarization direction. Normally in such systems without a polarization conversion system, half of the light is lost either from reflection or absorption. This results in a dimmer display or requires a brighter light source. In a projector, a dimmer display is more difficult to view, while a brighter light source increases the power consumption and the cost of the projector system. Due to the additional heat typically generated, the brighter light source may require a larger housing to provide enough room for cooling or to accommodate a fan to cool the light source. The fan adds additional cost, power consumption and noise. Polarization conversion systems have been developed to recover some of the lost light. However, the most commonly used polarization conversion systems use a multi-PBS (Polarizing Beam Splitter), which is expensive.
Recent developments in faster switching Liquid Crystal and Ferro-electric LC technologies have made single panel projection systems possible. In a single panel projection system, polarized light is needed as well as sequential or scrolling color. In a field sequential single panel projection system, the display is illuminated with short bursts of red, green, and blue light while the display is synchronized to the pulsed light source to reflect the appropriate color component of the image. A white light or other color light burst can also be used alone or in combination with the red, green, and blue light. The short bursts can come from a color wheel or from pulsed LEDs (Light Emitting Diodes). The rapidly alternating red, green, and blue images are blended in human perception to form the full-color image of the display. A field sequential system requires a bright single color light source for each burst. Such light sources are typically not very bright or are very inefficient.
In a scrolling or row sequential single panel system, the colors can be scrolled across the display and the display updates one row at a time instead of updating the entire frame. This approach is particularly useful with some liquid crystal displays that have slow response times. Color wheels and drums have been developed to provide bands of color that scroll across the display as the rows of the display are updated. These use color filters to transmit light of only one color, typically red, green or blue. The rest of the light, about two-thirds, is then lost, again reducing the brightness of the display. Again a brighter light source is required to compensate for the losses. In another system, a complex combination of prisms and filters is used to scroll colored light onto a display and then redirect that light to a projection screen. This system does not lose as much light, but requires a complex precisely aligned and synchronized rotating prism system.